The present invention relates generally to refrigeration and is particularly concerned with systems wherein products to be frozen, such as foods, are moved continuously through a treating tunnel while being contacted with cryogenic coolant.
Apparatus for continuous cooling and freezing of products, particularly food and the like, are well known in the art as exemplified, for example, by U.S. Pat. No. Re. 28,712; U.S. Pat. Nos. 3,403,527; 3,613,386; 3,813,895; 3,892,104; 4,229,947; which are assigned to the assignee of the present invention. Such apparatus usually includes an elongated tunnel defined by insulated walls and an endless conveyor belt extending longitudinally of the tunnel for moving articles therethrough. A cryogenic fluid, such as liquid nitrogen (LIN) is introduced as a spray into the tunnel, usually near the products exit end thereof. In a typical operation the liquid coolant is sprayed directly onto the product on the conveyor and is thereby vaporized by heat exchange therewith and is induced to flow through the tunnel as a vapor in counterflow relation to the movement of products on the conveyor, and is discharged near the products inlet end of the tunnel.
Systems of the type described, when properly operated under precise control, are highly efficient in utilization of coolant but are relatively costly. A sophisticated gas flow control system is required to pump the cold nitrogen gas toward the tunnel entrance. The volume of cold gas moved must exactly match the volume that is generated by vaporization of the coolant in the spray zone. If the pumped volume is too low, the excess very cold nitrogen gas spills out of the products discharge end of the tunnel, wasting about half of the available refrigeration. If the pumped volume is too high, warm room air will be pulled into the products discharge end of the tunnel, causing a large heat loss and frost accumulation. The gas flow control system requires a steady flow of coolant to function properly. Accordingly, the coolant control system must be provided with a proportioning controller and a motorized coolant supply valve to modulate flow of the coolant. This type of control system, manifestly, is more expensive, more complicated and more difficult to maintain than a simple "on-off" flow-control system.
Another disadvantage found in freezers of the type described, is their sensitivity to two-phase flow. As liquid nitrogen flows through a transfer line from the supply source, the pressure is lowered and heat enters through the insulation. These factors cause a portion of the coolant to vaporize, thereby forming a two-phase mixture of liquid and gas. In some cases, the liquid and gas segregate into slugs of gas followed by slugs of liquid. Such slug flow is very detrimental to the operation of the freezer. When the slug of coolant gas enters the spray header, the direct contact spray of liquid coolant is lost. Since direct spray of liquid coolant on the products provides about one-half of the refrigeration in these systems, the product passing under a gas-filled spray header will not be cooled sufficiently. Thus, when slug flow conditions occur, the product will be cooled erratically and incompletely.
The foregoing problems are not encountered in other systems wherein the product to be frozen is immersed in the cryogenic liquid coolant. Such systems comprise an insulated tank filled with LIN or other cryogenic liquid coolant, and a conveyor belt arranged to dip the conveyed product into the liquid. Such immersion freezer utilizes the latent heat of the liquid coolant but discards the very cold gas formed by the contact vaporization. The exhaust gas temperature of a typical LIN immersion freezer has been measured to be about -280.degree. F. (-173.degree. C.). Although such immersion freezers are of simple construction, easy to operate and occupy relatively little floor space, they are very inefficient with respect to utilization of coolant.
The coolant efficiency of any alternate freezing system can be compared with the heretofore described system of the direct spray type to establish their relative freezing costs. Assuming that liquid nitrogen (LIN) is employed as the coolant:
E.sub.LIN =(Q.sub.A /Q.sub.L) 100 PA1 Q.sub.A =available refrigeration of LIN warmed to the nitrogen gas discharge temperature, Btu/lb. LIN PA1 Q.sub.L =availabe refrigeration of LIN warmed to the incoming product temperature, Btu/lb. LIN PA1 Q=81.0+0.252 (320+T), Btu/lb. LIN;
wherein E.sub.LIN =LIN effeciency, %
For a LIN storage tank pressure of 17.5 psig
where T=Nitrogen gas temperature, .degree.F. (81.0 is the latent heat of liquid nitrogen at the storage pressure and 0.252 is its specific heat).
For an immersion type LIN freezer: ##EQU1##
For a freezing system of the type hereinbefore described utilizing direct spray of LIN on the product conveyed through the freezing tunnel and counterflow of vaporized LIN toward the product entrance end; wherein the products are brought from an inlet temperature of +100.degree. F. to a discharge temperature of 30.degree. F. with a nitrogen gas discharge temperature of +20.degree. F., the LIN efficiency is: ##EQU2##
The highest LIN efficiency is achieved in systems employing counterflow heat exchange between coolant and product cooled. However, this high LIN efficiency can be had and maintained only by carefully controlled operating practices and adequate maintenance of the equipment.
Among the objects of the present invention are to provide a simple, continuous cryogenic freezer of relatively low cost that can freeze products, such as foods or the like, economically and with relatively little sacrifice in LIN efficiency.